6″ DigiMag DRO scale

Introduction to the iGaging DROs

iGaging is a manufacturer of low-cost  measurement devices.  One of their more popular products has been the Digital Read Out unit’s called DigiMag.  While these readouts are rarely used in industrial or professional machine shops, they have found a very popular niche in the hobbyist and light machine shop.  Due to their size, low-cost and reliability, these units have become the go-to standard for small machinists all over the world.  The iGaging DRO has become so well accepted in this market space that even companies like Shars appear to be OEMing them and selling the units under the Shars brand.  If you are just getting into machining and are unaware of Shars.com, they are perhaps my favorite tool store!  Great prices and lots of selection, check them out.

Grounding and Noise Issues

One of the biggest complaints that people have with the iGaging DRO’s is that they are susceptible to electrical noise and grounding issues when attached to a mill or lathe.  There are many forum posts on the various machinist forums regarding the DRO randomly changing measurements and bouncing around between several thousandths.  Often, this behavior only occurs when the mill or lathe machine is turned on.  For those of us with an electronics background, we immediately recognize that this issue may be related to electrical noise and grounding issues.

Electrically noisy machines

The Mill and Lathe are very often, electrically noisy machines.  There are a number of sources of noise on a machine like the mill or lathe.

The motor:

The motor on a mini lathe or mini mill is often a brushed type motor.  A brushed motor throws out RF noise with each rotation of the rotor and each connect/disconnect cycle of the internal brush and armature contacts.  The sparks generated by the brush and armature contacts create wideband RF and electrical noise.

Reducing the amount of RF noise from the motor can be accomplished in several manners:

  1. Ensure that the motor housing is well grounded to the frame of the machine.  Make sure that there is no paint to insulate the motor housing from the mill head or lathe body.  If there is, either scrape away the paint from the mating surfaces, or run a braided grounding wire from the motor to the mill or lathe body.
  2. Add a clip-on Ferrite bead to the power cable that runs from the controller to the motor.  This will dissipate much of the electrical noise between the controller and the motor.
  3. In a severe case of noise, the motor can be surrounded by copper wire mesh which is then grounded.

Variable Speed Drive:

Another source of noise on the mill or lathe is the variable speed controller.  While older machines may not be variable speed, many of the newer units are.  Variable speed controllers often use a high-frequency Pulse Width Modulator (PWM) circuit to drive the motor at the desired speed while maintaining reasonable torque.  The PWM circuit utilizes a “chopper” to rapidly turn the voltage to the motor on and off many times per second.  In short, PWM works as follows for a motor controller: The more time during a second that the voltage is ON, the faster the motor turns.  The more time that the voltage is OFF, the slower the motor turns.  The process of “chopping” the voltage, or switching it on and off, generates electrical and RF noise at the speed of the controllers chopping circuit.  Additionally, there are harmonics that will also be generated that are above and below the PWM frequency.

Reducing the amount of electrical and RF interference from the controller may be accomplished by:

  1. Adding a Add a clip-on Ferrite bead to the power cable that runs from the controller to the motor.  This will dissipate much of the electrical noise between the controller and the motor.
  2. Add a clip-on ferrite bead to the the service power cable to reduce noise travelling from the machine back to the service power.
  3. Ensure that the controller is housed in a shielded box.  In short, this basically means a metal box.  While many of the mini-lathe and mini-mill platforms use plastic boxes, there are spray on shielding paints that can be applied to the inside of the plastic case.  Once shielded, ensure that that case is electrically grounded to the frame of the machine.

Mating of parts:

The mill and lathe are not one piece of solid metal.  They are made of 100’s of little parts including the base, bed, ways, table, cross-slide, head, feet, etc…  All of these metals parts conduct electricity and have an electrical potential.  Ensuring that all of the parts on the mill or lathe are at the same potential is important as it reduces electrical noise when the parts are moved against one another.  The problem is not that the parts are metal, and therefore conductive.  The problem comes in the interface, or mating point, of these parts.  It is important to ensure that each part is solidly attached at it’s mating interface to the next part otherwise and difference in electrical potential may develop between the parts.  This difference in potential between parts then plays into the grounding of the system as a whole.  If the DRO scale (slide) is electrically connected to the bed of the lathe, and the DRO sensor is electrically connected to the carriage, and they are not at the same electrical potential, then a problem can develop with the DRO’s ability to accurately receive the very low voltage digital signals from the sensor.

The primary method for reducing the difference in electrical potential of machine components can be one of the trickiest changes.

  1. Ensure that the parts are electrically connected.  If there is paint on mating surfaces, either remove it, or attach a braided ground wire between the components.
  2. All electrical grounding braids should connect to the same point on the machine.  Don’t “daisy-chain” the braid as this will result in more electrical potential differences.

Electrical Power Cables:

The electrical cables that provide the operational power to the machine are also important to take note of.  In North America the electrical grid operates at 60Hz and 50Hz in many other regions of the world.  The alternating current produces a source of noise that emanates from the power cables themselves.

To reduce the amount of electrical and EMF noise imposed ont he machine by the power cables:

  1. Ensure that the power cables do not run parallel to any metal surfaces on the machine.  If possible, the power cable should run perpendicarly away from the machine as soon as it leaves the machine controller box.
  2. Ensure that the power cables do not run parallel or near any signal wires for the machine.  Signal wires include DRO cables and other motor cables.